The present invention relates to a process for producing a natural rubber for a pressure-sensitive adhesive, and a natural rubber-based pressure-sensitive adhesive composition comprising the natural rubber obtained by the process.
Natural rubber-based pressure-sensitive adhesives are generally prepared by dissolving a natural rubber in an organic solvent such as toluene, hexane or heptane, and then dissolving therein a compounding additive such as a tackifier resin or a softening agent. However, an unmasticated natural rubber generally has very high molecular weight, and for this reason, it is difficult to directly dissolve the unmasticated natural rubber in an organic solvent. Further, if a natural rubber has too high molecular weight, the natural rubber lacks flexibility when used for a pressure-sensitive adhesive, and as a result, there is the possibility that appropriate pressure-sensitive properties may not be obtained. Therefore, in order to avoid those problems, it is necessary to appropriately decrease a molecular weight of a natural rubber and use such for blending.
A method for decreasing a molecular weight of a natural rubber is generally to previously masticate mechanically with a pressure kneader, a Banbury mixer, an open roll or the like prior to dissolving the same in an organic solvent.
However, this mechanical mastication step to adjust a molecular weight of a natural rubber consumes a huge energy. Further, because a natural rubber has a self-fusion property, the mastication step itself undergoes very large working load. In addition, load is very large particularly in handling or transportation of a rubber after mastication, other than the mastication step itself.
For example, a natural rubber just after mechanical mastication is soft, so that a self-fusion by contact tends to occur. If natural rubber is fused to form a large bulk, this results in great problem on works in the subsequent step. In order to avoid this problem, countermeasures are generally taken to form a natural rubber into predetermined shape and size and spray a powder for self-fusion prevention thereon, or to interpose a release sheet therebetween. However, there are many problems in spraying such a powder or use such a release sheet. For example, where a powder is used, working environment deteriorates by dust, and also a product is contaminated by such a powder in the form of an impurity. Further, where a release sheet is used, it is troublesome to lay many sheets, and also a sheet after use becomes an industrial waste. Furthermore, an unmasticated natural rubber adheres relatively strong to a release sheet, much labor may be required to peel the sheet in the subsequent step.
Therefore, the mastication step involves very disadvantageous elements in production cost, working standpoint and environmental standpoint, and improvement has been demenaded in the mastication step.
Further, as an environmental countermeasure, where low viscosity and high solid content concentration are intended for the purpose of decreasing the amount of an organic solvent used in blending a natural rubber with a pressure-sensitive adhesive, it needs to greatly decrease a molecular weight of a natural rubber. The conventional method involves the disadvantages that the necessity causes to conduct excess mechanical mastication, so that load in the mechanical mastication step further increases than the present.
On the other hand, a method that the degree of mastication is minimized or mastication time is shortened is considered as a method for reducing load of mechanical mastication step as possible. However, those methods have the limitation only that a molecular weight of a natural rubber is decreased to a weigh average molecular weight Mw of about 700,000 to 1,000,000, and a natural rubber having further decreased molecular weight cannot be obtained. Where a natural rubber has a molecular weight of this level, such a natural rubber satisfies properties as a pressure-sensitive adhesive tape. However, where the natural rubber is dissolved in an organic solvent, the resulting solution has considerably high viscosity, and this makes it difficult to uniformly apply a pressure-sensitive adhesive to a substrate in a small thickness in producing a pressure-sensitive adhesive tape. In producing a pressure-sensitive adhesive tape, it is preferable to decrease a molecular weight of a natural rubber to a weight average molecular weight of 700,000 or less in order to suppress a solution viscosity of a pressure-sensitive adhesive low, considering the amount of an organic solvent used.
Accordingly, one object of the present invention is to provide a process that can efficiently produce a natural rubber for a pressure-sensitive adhesive with a simple operation without passing through complicated steps.
Another object of the present invention is to provide a process that can simply and easily produce a natural rubber for a pressure-sensitive adhesive, having a desired appropriate molecular weight.
Still another object of the present invention is to provide a natural rubber-based pressure-sensitive adhesive composition that can decrease its molecular weight even though having high solid content concentration.
As a result of extensive investigations to achieve the above objects, it has been found that if a specific treatment is applied to an unmasticated natural rubber, the molecular weight of the natural rubber can easily be decreased to the desired molecular weight without passing through a mechanical mastication step, and as a result, a natural rubber solution having low viscosity even though at high solid content concentration can easily be obtained, making it possible to greatly decrease the amount of an organic solvent used. The present invention has be completed based on this finding.
The present invention provides a process for producing a natural rubber for a pressure-sensitive adhesive, which comprises decreasing a molecular weight of an unmasticated natural rubber by the action of a peptizing agent while dissolving the unmasticated natural rubber in an organic solvent.
The present invention also provides a natural rubber-based pressure-sensitive adhesive composition comprising a natural rubber having a weigh average molecular weight Mw of 700,000 or less obtained by the process.
The present invention is described in detail below.
According to the present invention, the natural rubber for a pressure-sensitive adhesive is obtained by decreasing a molecular weight of an unmasticated natural rubber by the action of a peptizing agent while dissolving the unmasticated natural rubber as is in an organic solvent without controlling the molecular weight in a mechanical mastication step.
Examples of the natural rubber that can be used include standard malaysian rubber (SMR), standard vietnamese rubber (SVR), ribbed smoked sheet (RSS) Nos. 1 to 6 and pale crepe Nos. 1 to 3. However, the natural rubber that can be used is not limited to those so long as it is a general high molecular weight solid type natural rubber.
Examples of the solvent used include aromatic hydrocarbons such as toluene, and aliphatic hydrocarbons such as heptane or hexane. However, the solvent is not limited to those so long as it is an organic solvent that can dissolve a natural rubber.
The peptizing agent used herein means a reagent that can chemically cut a natural rubber molecule in an organic solvent. Preferable examples of the peptizing agent include disulfides such as o,o-dibenzamide diphenyldisulfide, and mercaptans such as 2-benzamide thiophenol zinc salt, 2-thionaphthol, thioxylenol or pentachlorothiophenol. Those compounds having a metal catalyst added thereto may also be used. Further examples of the peptizing agent that can be used include thiazoles such as 2-mercaptobenzothiazole, diacyl peroxides such as benzoyl peroxide, dialkyl peroxides such as dicumyl peroxide, and other organic peroxides.
Compounds containing reactive functional groups, such as hydroxyl group or carboxyl group, in a molecule of the above peptizing agent (hereinafter referred to as xe2x80x9cfunctional group-introducing peptizing agentxe2x80x9d) may be used as a specific peptizing agent. Those compounds may be used alone or as a mixture with the above-described general peptizing agents at an optional mixing ratio. When a natural rubber is masticated using such a functional group-introducing peptizing agent, the reactive functional groups can be introduced into the natural rubber molecular chain.
Of the functional group-introducing peptizing agents, examples of compounds containing hydroxyl group include 2-hydroxydiphenyl disulfide, 2-hydroxyethyl disulfide, mercaptoethanol, and 3-mercapto-1,2-propanediol. However, the functional group-introducing peptizing agents are not limited to the above compounds, and any compounds can be used so long as it has a structure such that hydroxyl group is contained in the molecule of the above-described general peptizing agents.
Of the functional group-introducing peptizing agents, examples of compounds containing mercapto group include mercaptobenzoic acid, mercaptoacetic acid and mercaptopropionic acid. However, the functional group-introducing peptizing agents are not limited to the above compounds, and any compounds can be used so long as it has a structure such that mercapto group is contained in the molecule of the above-described general peptizing agents.
Carboxylic anhydrides such as maleic anhydride may also be used as the functional group-introducing peptizing agent. When maleic anhydride, for example, and the above-described peptizing agent are used in combination at optional mixing ratio, acid anhydride group can be introduced into a natural rubber molecular chain.
The natural rubber for a pressure-sensitive adhesive is obtained by introducing, for example, a natural rubber, an organic solvent, a peptizing agent and if necessary, appropriate additives in a dissolution tank at predetermined blending ratio, and stirring the contents at a predetermined temperature for a predetermined time. The dissolution tank used is not particularly limited so long as it is generally used in a dissolution step.
Molecular weight of the natural rubber can be controlled to the desired value by adjusting the amount of the peptizing agent introduced. The peptizing agent may be introduced together with the natural rubber at the initial charging, or may be introduced at optional timing. The peptizing agent may further be introduced such that the peptizing agent is divided into two portions, one portion is introduced at the initial stage, and another portion is introduced in the course of reaction. Molecular weight of the natural rubber can also be controlled to the aimed molecular weight by the timing for introducing the peptizing agent.
Reaction temperature can appropriately be selected depending on the kind of peptizing agent used and the aimed molecular weight of natural rubber. The reaction temperature is generally 70xc2x0 C. or higher, preferably 80 to 120xc2x0 C. Stirring and reaction time is a period up to that the natural rubber is completely dissolved to disappear all undissolved solid contents, and further molecular weight of the natural rubber can be decreased to the predetermined molecular weight.
According to the process of the present invention, an unmasticated natural rubber having high molecular weight, which is a solid at the initial stage, is dissolved in an organic solvent, and simultaneously the molecules thereof are chemically cut by a peptizing agent, heat energy and the like, whereby a solution of the natural rubber having the desired molecular weight (for example, weight average molecular weight Mw is 700,000 or less), which is suitable to a pressure-sensitive adhesive, can be prepared.
It is known that oxygen is generally necessary in peptizing reaction of a natural rubber. Peptizing reaction can be promoted in the present invention by blowing air in a reaction system. Where the process of the present invention is conducted at high solid content concentration, it is necessary to greatly decrease molecular weight. To this effect, it is preferable to blow air.
Viscosity of the natural rubber solution prepared in the present invention can freely be adjusted by controlling molecular weight of the natural rubber using a peptizing agent, a reaction temperature, a reaction time, an oxygen concentration or the like as parameters. For example, when the viscosity is adjusted to 80 Paxc2x7s or less, preferably 40 Paxc2x7s or less, in terms of Brookfield type viscometer 30xc2x0 C. viscosity, such a natural rubber can directly be utilized as a pressure-sensitive adhesive composition.
The natural rubber-based pressure-sensitive adhesive composition of the present invention comprises a natural rubber having a weight average molecular weight Mw of 700,000 or less (for example, 200,000 to 700,000) obtained by the process of the present invention.
To uniformly apply the natural rubber-based pressure-sensitive adhesive composition on a substrate at a small thickness in a pressure-sensitive adhesive tape production step, it is desirable that the viscosity thereof is 80 Paxc2x7s or less, preferably 40 Paxc2x7s or less, in terms of Brookfield type viscometer 30xc2x0 C. viscosity. If weight average molecular weight Mw of the natural rubber in the pressure-sensitive adhesive exceeds 700,000, the solid content concentration must be 20% by weight or less in order to make the solution viscosity 40 Paxc2x7s or less. This needs a large amount of an organic solvent, which is not preferable.
The natural rubber-based pressure-sensitive adhesive composition can be produced by, if necessary, adding a tackifier resin, or conventional additives such as a crosslinking agent, softening agent, an antioxidant, a filler or the like, to the natural rubber solution obtained by the above-described process, and if necessary, adjusting a concentration of the resulting solution.
The tackifier is not particular limited, and examples thereof include rosin resins, terpene resins, petroleum resins (aliphatic, aromatic or alicyclic), coumarone-indene resins and styrenic resins. The amount of the tackifier used is generally about 5 to 180 parts by weight, preferably 70 to 130 parts by weight, per 100 parts by weight of the natural rubber.
Solid content concentration of the natural rubber-based pressure-sensitive adhesive composition is automatically determined by the aimed solution viscosity and molecular weight of a natural rubber. In view of the worldwide environmental problem, it is necessary to decrease the amount of a solvent used as less as possible, and from this point the solid content concentration is desirably 20% by weight or more (for example, about 20 to 75% by weight).
In the above-described process, decreasing molecular weight of the natural rubber after peptizing can achieve low viscosity and high solid content concentration, and as a result, can contribute to reduction in amount of an organic solvent used. Where such low viscosity and high solid content concentration are intended, it is preferable to use the above-described functional group-introducing peptizing agent. In general, if molecular weight of the natural rubber decreases, cohesive force of a pressure-sensitive adhesive tends to lower. However, if functional group is introduced into the natural rubber molecule using the functional group-introducing peptizing agent, the molecular chain can efficiently be extended by a crosslinking agent even though being considerably low molecular weight. As a result, pressure-sensitive adhesive tape properties closely near those of a high molecular weight product can be realized regard a cohesive force or the like.
The present invention is described in more detail by reference to the following Examples, but it should be understood that the invention is not construed as being limited thereto. Unless otherwise indicated, all parts, percents and the like are by weight.